A. Platelets
Platelets are particles found in whole blood that initiate and provide the structural basis for the hemostatic plug necessary to stop bleeding. Platelets depend on adhesive interactions with extracellular proteins and other cells for proper function (see Hawiger, J. Atherosclerosis Reviews 21:165-186 (1990) and Roth J. R. Immunology Today 13(2):100-105 (1992The external platelet plasma membrane surface is covered with a variety of membrane bound glycoproteins, many of which have recognition and adhesive functions. Perhaps the most abundant platelet membrane adhesive proteins belong to the integrin superfamily which include the glycoproteins; GPII.sub.b III.sub.a, GPI.sub.a II.sub.a, GPI.sub.c II.sub.a, GPI.sub.b IX, and the fibronectin and vitronectin receptors (Hynes, R. O., Cell, 48:549 (1987). Each integrin receptor is an .alpha..beta. heterodimer displaying characteristic affinity and specificity toward various protein ligands found in serum and/or the extracellular matrix including; von Willebrand factor (vWF), collagen, entactin, tenascin, fibronectin (Fn), vitronectin (Vn), and laminin, as well as fibrinogen (Fg) and thrombospondin (see Kieffer et al., Ann. Rev. Cell Biol. 6:329-357(1990) and Ruoslahti, J. Clin. Invest., 87:1-5 (1991)). The most abundant integrin found on the surface of normal platelets is GPII.sub.b III.sub.a comprising about 50,000 molecules per platelet and representing about 2% of the total platelet protein. GPII.sub.b III.sub.a is a non-covalent, calcium ion dependent heterodimeric complex (Jennings, et al., J. Biol. Chem. 257: 10458 (1982)) that is restricted in distribution to platelets and other cells of the megakaryocytic lineage (Kieffer et al., supra). On activated platelets, GPII.sub.b III.sub.a promiscuously binds a number of protein ligands with varying affinities, including; fibrinogen, fibronectin, von Willebrand factor, vitronectin and thrombospondin (Plow et al., Biochemistry of Platelets, Phillips and Shuman eds., p. 225-256, Orlando: Academic Press [1986]). Each of these protein ligands contain at least one tripeptide sequence Arg-Gly-Asp (RGD) which is commonly referred to as the "recognition sequence". It is believed the most important interactions mediating platelet aggregation involve GPII.sub.b III.sub.a binding with the trinodular fibrinogen and, to a lesser extent, with the filamentous von Willebrand factor (Kieffer et al., supra and Albeda et al., The FASEB Journal, 4:2868-2880 [1990]).
GPII.sub.b III.sub.a binding to its natural ligands can be inhibited to varying degrees by peptides and proteins containing the amino acid recognition sequences; Arg-Gly-Asp (Ruoslahti, supra and EPO 368 486, assigned to Merck & Co.), Lys-Gly-Asp (KGD), and the fibrinogen .gamma.-chain carboxy-terminal dodecapeptide HHLGGAKQAGDV and analogues thereof (Timmons et al., Biochemistry, 28:2919-2922 [1989]).
B. The Hyperthrombotic State
Many common human disorders are characteristically associated with a hyperthrombotic state leading to intravascular thrombi and emboli. These are a major cause of medical morbidity, leading to infarction, stroke and phlebitis, and of mortality from stroke and pulmonary and cardiac emboli. Patients with atherosclerosis are predisposed to arterial thromboembolic phenomena for a variety of reasons. Atherosclerotic plaques form niduses or platelet plugs and thrombi that lead to vascular narrowing and occlusion, resulting in myocardial and cerebral ischemic disease. This may happen spontaneously or following procedures such as angioplasty or endarterectomy. Thrombi that break off and are released into the circulation cause infarction of different organs, especially the brain, extremities, heart and kidneys.
In addition to being involved in arterial thrombosis, platelets may also play a role in venous thrombosis. A large percentage of such patients have no antecedent risk factors and develop venous thrombophlebitis and subsequent pulmonary emboli without a known cause. Other patients who form venous thrombi have underlying diseases known to predispose them to these syndromes. Some of these patients may have genetic or acquired deficiencies of factors that normally prevent hypercoagulability, such as antithrombin-3. Others have mechanical obstructions to venous flow, such as tumor masses, that lead to low flow states and thrombosis. Patients with malignancy have a high incidence of thrombotic phenomena for unclear reasons. Antithrombotic therapy in this situation with currently available agents is dangerous and often ineffective.
Patients whose blood flows over artificial surfaces, such as prosthetic synthetic cardiac valves or through extracorporeal perfusion devices, are also at risk for the development of platelet plugs, thrombi and emboli. It is standard practice that patients with artificial cardiac valves be treated chronically with anti-coagulants. However, in all instances, platelet activation and emboli formation may still occur despite adequate anticoagulation treatment.
Thus, a large category of patients, including those with atherosclerosis, coronary artery disease, artificial heart valves, cancer, and a history of stroke, phlebitis, or pulmonary emboli, are candidates for limited or chronic antithrombotic therapy. The number of available therapeutic agents is limited and these, for the most part, act by inhibiting or reducing levels of circulating clotting factors. These agents are frequently not effective against the patient's underlying hematologic problem, which often concerns an increased propensity for platelet aggregation and adhesion. They also cause the patient to be susceptible to abnormal bleeding. Available antiplatelet agents, such as aspirin, inhibit only part of the platelet activation process and are therefore often inadequate for therapy and also cause the patient to be susceptible to abnormal bleeding. An ideal anti-thrombotic drug would have many properties currently not available (see e.g. Sixma, et al. Thrombosis Research 67:305-311 [1992]).
C. Therapeutic Agents
An agent which effectively inhibits the final common pathway of platelet activation, namely fibrinogen binding to the GP II.sub.b III.sub.a receptor, should accordingly be useful in a large group of disorders characterized by a hyperthrombotic state as described above.
Such agents include anti-thrombotic peptides and pseudopeptides capable of inhibiting platelet aggregation. Ruoslahti et al. (U.S. Pat. No. 4,578,079) suggest that tetrapeptides containing the RGD sequence may be used to inhibit platelet aggregation. Zimmerman et al. (U.S. Pat. No. 4,683,291) disclose that positively charged amino acid residues (e.g. Arg and Lys) and homologues located before or toward the amino terminus of the RGD sequence are superior for inhibiting fibrinogen-platelet binding. Adams et al. (U.S. Pat. No. 4,857,508) describe superior results for in vitro inhibition of human platelet aggregation in platelet-rich plasma for linear tetrapeptides containing O-methyl-Tyr-amide immediately following the RGD (or homo-RGD) sequence. Barker et al., WO 90/01331, disclose substantially rigid RGD cyclic peptides possessing high affinity for the GP II.sub.b III.sub.a receptor. A particularly efficacious rigid RGD cyclic peptide described by Barker et al. is represented by the following structure: ##STR1## Tjoeng et al. (U.S. Pat. No. 4,879,313) describe peptide mimetic platelet aggregation inhibitors in which the first two residues of the RGD sequence are replaced by the pseudodipeptidyl 8-guanidino-octanoyl moiety. Other peptidomimetics in which the Arg of the RGD sequence has been altered include; WO89/076t19 (homo-Arg[Har]), EP 341 915 (Har and alkyl-Arg), WO90/15620 (Har and amidino derivatives e.g. imidazolinyl, imidazolyl, and substituted imidazolyl), EP 422 937 (aryl-, arylalkyl-, and cycloalkyl-amines), WO91/07976 (alkylamidino and alkylamino derivatives), and WO91/04247 (alkylamino and alkylguanidino proline derivatives). See also EP 384 362 (glycine derivatives) and EP 381 033.
Complete replacement of all residues in the RGD sequence has been described in EP 372 486, assigned to Hoffmann La Roche, where platelet aggregation inhibitors that are derivatives of benzoic and phenylacetic acid are presented. A benzoic acid derivative inhibitor having a particular low IC.sub.50 in an ELISA measurement of fibrinogen GPII.sub.b III.sub.a binding has the following structure: ##STR2## Fibrinogen receptor antagonists possessing similar structures can be found in EP 478 362, EP 478 363, and EP 478 328, all assigned to Merck. A representative Merck compound has the following structure: ##STR3## Also of interest are biphenyl derivatives described in EP 483 667 and EP 496 378, the latter publication providing a representative compound having the structure; ##STR4## Quinazoline-3-alkanoic acid derivatives are also reported to have an inhibitory effect on platelet aggregation (although possibly by a different mechanism) in EP 456 835 A1. A generic formula representing these compounds is given by: ##STR5## where n is 1 to 3 and R.sup.2 and R.sup.3 may be interalia hydrogen, lower alkyl, lower alkoxy, aralkyl groups that may be substituted with interalia --NR.sup.4 R.sup.5, where R.sup.4 or R.sup.5 may be hydrogen, lower alkyl, or connected with each other to make five- or six-membered heterocycles which may contain another heteroatom, and A--R.sup.1 may be lower alkyl.
D. Benzodiazepines
It is well established that benzodiazepines and related ligands interact with a specific site commonly referred to as the "benzodiazepine receptor" that is associated with a neuro-inhibitory postsynaptic GABA receptor and a chloride ionophore channel (see eg. Watjen et al., J. Med. Chem. 32:2282-2291[1989]). Binding of ligands to this receptor is known to produce a wide variety of neuro-physiological effects. Benzodiazepines have not been reported to have platelet aggregation inhibition activity. The preparation and therapeutic use of benzodiazepines is described in, for example; EP 0 059 390, EP 0 059 386, and EP 0 394 101.
Benzodiazepinediones have been employed as intermediates in the synthesis of various anti-HIV-1 compounds. For example Kukla, M. J. et al., J. Med. Chem. 34:3187-3197 (1991) reduce the dilactam to either the corresponding "-one" or diamine in the preparation of various TIBO derivatives having anti-HIV activity. Pyridodiazepines are also described as useful for treatment of HIV infection in U.S. Pat. No. 5,087,625.
E. Objects
It is an object of this invention to produce nonpeptidyl compounds having potent antithrombotic activity. It is another object of the invention to produce such compounds that are essentially free of amide bonds, substantially rigid, and stable to degradation. It is a further object to produce potent nonpeptidyl antithrombotics that specifically inhibit the GPII.sub.b III.sub.a -Fg interaction but do not strongly inhibit other RGD sensitive integrin interactions including the Vn-VnR, Fn-FnR, and GPII.sub.b III.sub.a -vWF interactions. It is still a further object to produce potent nonpeptidyl platelet aggregation inhibitors that do not significantly increase cutaneous bleeding time or diminish other hemodynamic factors. It is also an object of this invention to produce nonpeptidyl compounds having a long half-life and a large therapeutic range.
It is still a further object to produce nonpeptidyl compounds that are capable of inhibiting other integrin interactions such as the Vn-VnR interaction.
These and other objects of this invention will be apparent from consideration of the invention as a whole.